Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Pacemaker leads form the electrical connection between the cardiac pacemaker pulse generator and the heart tissue which is to be stimulated. As is well known, the leads connecting such pacemakers with the heart may be used for pacing, or for sensing electrical signals produced by the heart, or for both pacing and sensing in which case a single lead serves as a bi-directional pulse transmission link between the pacemaker and the heart or may be used to deliver defibrillation shocks to the patient. A typical transvenous type pacing/sensing lead, that is, a lead which is inserted into a vein and guided therethrough into a cavity of the heart, includes at its distal end an electrode designed to contact the endocardium, the tissue lining the inside of the heart. The lead further includes a proximal end having a connector terminal pin adapted to be received by a mating socket in the pacemaker. A flexible, coiled or cabled conductor surrounded by an insulating tube or sheath typically couples the connector terminal pin at the proximal end and the electrode at the distal end.
The implantable cardiac stimulation leads with which the present invention is concerned may take the form of pacemaker leads capable of pacing and sensing in at least one chamber of the heart. Indeed, the present invention, may relate to a programmable dual chamber pacemaker wherein the basic configuration of the pacemaker, e.g. unipolar or bipolar, can be changed, including the grounding configuration and ground potentials used within the pacemaker.
Generally, a heart stimulator, commonly known as a “pacemaker” or “pacer”, uses one, two, or more flexible leads having one end connected to the pacer and the other end connected to electrodes placed in close proximity to the heart. These leads are used to stimulate or pace the heart. These leads are also used to sense the heart's electrical activity by sensing the heart's signals from their electrodes.
In order to properly pace or sense, the pacer has to be able to deliver a stimulating pulse to the heart or sense an electrical signal from the heart, and this requires that there be an electrical return path. If, within a given heart chamber, a unipolar lead is used—containing a single conductor—the return path is the conductive body tissue and fluids. The return path is connected to the pacer by connecting the pacer electrical common or ground to the pacer metal enclosure, typically referred to as the pacer case or housing. The case, in turn, makes contact with the body tissue and/or fluids. Then the current flows from the pacemaker through the lead's conductor, then through the lead's electrode, then through tissue, and finally to the pacer case.
An alternative solution to using a unipolar lead in a given heart chamber is to use a double lead/electrode in the heart chamber, known as a bipolar lead. In a typical bipolar lead, a second conductor coil or cable is spiraled over or positioned in a separate lumen and insulated from a first conductor along the length of the lead. At the distal end of the lead, one of the conductor cables or coils (inner) is connected to a first electrode, referred to as the “tip” electrode, and the second (outer) conductor coil is connected to a second electrode, referred to as a “ring” electrode. The ring electrode is generally situated about 10 to 20 mm from the tip electrode. The tip electrode is typically placed in contact with heart tissue, while the ring electrode is likely to be mostly in electrical contact with the blood. Because both body tissue and fluids are conductive, the ring electrode of a bipolar lead, in contact with the body fluids, serves as an electrical return for both pacing and sensing.
As earlier noted, the present invention has application to placing leads whose electrodes are either endocardial usage or epicardial usage. In the customary manner, a transvenous lead with an endocardial electrode provides an electrical pathway between the pacemaker, connected to the proximal end of the lead, and endocardial tissue in contact with the distal end of the lead. Endocardial tissue refers to a specific layer of tissue in the interior of the heart's chambers. In such a manner electrical pulses emitted by the pacemaker travel through the endocardial lead and stimulate the heart.
Transveous leads with endocardial electrodes are often placed in contact with the endocardial tissue by passage through a venous access, such as the cephalic or subclavian veins or one of their tributaries. In such a manner transvenous leads offer as an advantage that their electrodes may be placed into contact with the heart without requiring major thoracic surgery. Rather, transvenous leads may be introduced into a vein and maneuvered therefrom so that their electrodes make contact with the endocardium of the heart.
A multi-step procedure is often used to introduce such leads within the venous system. Generally this procedure consists of inserting a hollow needle into a blood vessel, such as the subclavian vein. A wire guide is then passed through the needle into the interior portion of the vessel. The needle is then withdrawn and a delivery sheath and dilator assembly is then inserted over the wire guide into the vessel. The assembly is advanced into a suitable position within the vessel, i.e. so that the distal end is well within the vessel but the proximal end is outside the patient. Next, generally, the dilator and wire guide are both removed, although sometimes the wire guide is retained in place, in case it is needed again. The delivery sheath is left in position and through its hollow lumen offers direct access from outside the patient to the interior of the blood vessel. The ultimate positioning of the lead is often performed with the aid of a stylet unit. The stylet unit has an elongated main member and in its known configuration has a rigid knob at its proximal end for manipulating an enlarged distal tip end engageable with a thrusting region of the lead. In such a fashion a transvenous lead can be easily passed into the vessel through the delivery sheath and ultimately be positioned within the heart. Finally the delivery sheath is removed from the body. With respect to pacemaker leads, however, which typically have a relatively bulky connector pin assembly at the proximal end, the delivery sheath is removed from the body by being split or slit apart. In such a manner the delivery sheath does not have to be large enough to be removed over the relatively bulky connector pin assembly at the proximal end of the lead.
There is an added problem, however, in that the proximal end of the sheath has an integral valve with a lumen and a bore dimensioned smaller than the knob of the stylet unit. The present invention addresses this situation. Typical of the known prior art are U.S. Pat. No. 4,498,482 to Williams, and U.S. Pat. Nos. 5,728,148 and 5,807,339 to Boström et al. which disclose stylet constructions with a ball at the distal end. U.S. Pat. No. 4,796,642 to Harris discloses a stylet with a distal region which includes, successively, a decreasing taper, a flat, an increasing taper, and a tip. U.S. Pat. No. 6,623,480 to Kuo et al. discloses an electrode catheter with a body constructed of PTFE and is electrically conductive. U.S. Patent Application Publication 2002/0077583 to Clemens et al. a catheter with a stylet lumen that collapses upon removal of the stylet. U.S. Patent Application Publications US 2002/0165536 to Kelley et al. and US 2002/0173785 to Spear et al disclose implantable systems in which the delivery sheath and rotatable hemostatic valves are splittable for removal around a manipulating knob.
It was in light of the foregoing that the present invention was conceived and has now been reduced to practice.